'\" te
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.\"  This file and its contents are supplied under the terms of the
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.TH EPOLL 7 "May 16, 2020"
.SH NAME
epoll \- Linux-compatible I/O event notification facility
.SH SYNOPSIS
.nf
#include <sys/epoll.h>
.fi

.SH DESCRIPTION
\fBepoll\fR is a facility for efficient event-oriented I/O that has a
similar model to \fBpoll\fR(2), but does not necessitate rescanning a
set of file descriptors to wait for an event.  \fBepoll\fR is of Linux
origins, and this facility is designed to be binary-compatible with
the Linux facility, including the following interfaces:

.RS +4
.TP
.ie t \(bu
.el o
\fBepoll_create\fR(3C) creates an \fBepoll\fR instance, returning a file
descriptor. It contains a size argument which is meaningful only in as
much as it cannot be 0.
.RE
.RS +4
.TP
.ie t \(bu
.el o
\fBepoll_create1\fR(3C) also creates an \fBepoll\fR instance, but eliminates
the meaningless size argument -- replacing it instead with a flags
argument.
.RE
.RS +4
.TP
.ie t \(bu
.el o
\fBepoll_ctl\fR(3C) allows file descriptors to be added
(via \fBEPOLL_CTL_ADD\fR), deleted (via \fBEPOLL_CTL_DEL\fR) or
modified (via \fBEPOLL_CTL_MOD\fR) with respect to the \fBepoll\fR'd set
of file descriptors.
.RE
.RS +4
.TP
.ie t \(bu
.el o
\fBepoll_wait\fR(3C) fetches pending events for file descriptors added
via \fBepoll_ctl\fR(3C), blocking the caller if no such events are pending.
.RE
.RS +4
.TP
.ie t \(bu
.el o
\fBepoll_pwait\fR(3C) operates in a similar manner to \fBepoll_wait\fR(3C), but
allows the caller to specify a signal mask to be set atomically with respect
to waiting for events.
.RE

.sp
.SH NOTES
The \fBepoll\fR facility is implemented
for purposes of offering compatibility to and portability of Linux-borne
applications; native applications should continue to prefer using event ports
via the \fBport_create\fR(3C),
\fBport_associate\fR(3C) and \fBport_getn\fR(3C) interfaces.
In particular, use of \fBepoll\fR in a multithreaded environment is fraught
with peril; even when using \fBEPOLLONESHOT\fR for one-shot events,
there are race conditions with respect to \fBclose\fR(2) that are unresolvable.
(For more details, see the aborted effort in Linux to resolve this via the
proposed
\fBEPOLL_CTL_DISABLE\fR operation.)
The event port facility -- like the BSD kqueue facility that inspired it --
is designed to deal with such issues via explicit event source dissociation.

While a best effort has been made to mimic the Linux semantics, there
are some semantics that are too peculiar or ill-conceived to merit
accommodation.  In particular, the Linux \fBepoll\fR facility will -- by
design -- continue to generate events for closed file descriptors where/when
the underlying file description remains open.  For example, if one were
to \fBfork\fR(2) and subsequently close an actively \fBepoll\fR'd file
descriptor in the parent,
any events generated in the child on the implicitly duplicated file descriptor
will continue to be delivered to the parent -- despite the fact that the
parent itself no longer has any notion of the file description!
This \fBepoll\fR facility refuses to honor
these semantics; closing the \fBEPOLL_CTL_ADD\fR'd file descriptor
will always result in no further
events being generated for that event description.

.SH SEE ALSO
.BR epoll_create (3C),
.BR epoll_create1 (3C),
.BR epoll_ctl (3C),
.BR epoll_pwait (3C),
.BR epoll_wait (3C),
.BR port_associate (3C),
.BR port_create (3C),
.BR port_dissociate (3C),
.BR port_get (3C),
.BR pselect (3C)
